1. Field of the Invention
The present invention relates to: a toner for forming an image through electrophotography; and an image forming apparatus, an image forming method, and a process cartridge each of which uses the toner.
2. Description of the Related Art
In recent years, toners for use in electrophotography have been demanded to be fixed at lower temperatures. This demand results from energy saving achieved by reducing energy for fixing and also from requirements for increasing process speed and image quality of electrophotographic image forming apparatus.
In general, as the process speed of electrophotographic image forming apparatus is increased, the image quality thereof is degraded. The reasons why this phenomenon occurs depend on various factors. However, among them, the greatest factor is an effect of fixing failures in a fixing step of an image forming process.
In the fixing step, a prefixed toner image on a recording medium typified by paper is fixed on the recording medium by heat and pressure to be a fixed toner image. In a rapid system speed, a prefixed toner image cannot receive a sufficient amount of heat in the fixing step. As a result, fixing failures occur to potentially roughen the surface of the finally obtained toner image or cause afterimage formation called cold offset, leading to failure images. In one possible measure to avoid fixing failures, a fixing temperature is increased in accordance with an increase in the system speed in order to keep image quality. Such an increased fixing temperature is not necessarily the best measure from the viewpoints of: adverse side effects of leakage of heat from a fixing device on other processes in the image forming apparatus; acceleration of abrasion speed of fixing members; and increase in consumption energy.
In view of this, particularly in high-speed image forming apparatus, improvements in fixing performance of toners themselves have been required. More specifically, demand has arisen for toners exhibiting sufficient fixing property at lower temperatures in the fixing step.
Conventionally, various attempts have been made to improve fixing property of toners. In one known method for improving fixing property of toners, binder resins contained in toners are controlled in terms of thermal characteristics typified by glass transition temperature (Tg) and softening temperature (T1/2). However, a resin having a lowered Tg causes degradation of heat resistance storageability, and a drop in T1/2 of a resin, which is attained by reducing its molecular weight, raises problems such as occurrence of hot offset. Therefore, only controlling thermal characteristics of a resin itself is not enough to obtain a toner satisfactory in all of low-temperature fixing property, heat resistance storageability and hot offset resistance.
In proposals of changing the kind of a binder resin for responding to fixing at lower temperature, widely-used conventional stryene-acryl resins are changed to polyester resins excellent in low-temperature fixing property and better in heat resistance storageability (see Japanese Patent Application Laid-Open (JP-A) Nos. 60-90344, 64-15755, 02-82267, 03-229264, 03-41470 and 11-305486). In another proposal, a binder is supplemented with a specific non-olefin crystalline polymer exhibiting sharp melt property at its glass transition temperature to improve low-temperature fixing property (see JP-A No. 62-63940). In these proposals, however, it cannot be said that resultant toners are optimized in molecular structure and molecular weight. There is a problem that they do not have sufficient low-temperature fixing property.
In addition, it has been proposed to use a crystalline polyester having sharp melt property similar to the above specific non-olefin crystalline polymer in a toner in order to improve its fixing property (see Japanese Patent (JP-B) No. 2931899 and JP-A No. 2001-222138). In this proposal, however, the acid value and the hydroxyl value of the crystalline polyester used in the toner are as low as 5 mgKOH/g or lower and 20 mgKOH/g or lower, respectively. The crystalline polyester has low affinity to paper, raising a problem that the toner does not have sufficient low-temperature fixing property.
In this proposal, optimization is not made on the molecular weight of the finally obtained toner and the existing state of the crystalline polyester. Therefore, toners obtained using the crystalline polyester of this proposal cannot sufficiently exhibit excellent low-temperature fixing property or heat resistance storageability attributed to the crystalline polyester, which is problematic. Also, it does not respond to requirements on hot offset resistance, raising a problem that a range of temperature for good image fixing cannot be ensured.
In one proposed method for controlling the existing state of the crystalline polyester, a crystalline polyester resin and a non-crystalline polyester resin, which are not compatible, are used to establish a sea-island, phase-separated structure (see JP-A No. 2004-46095). This proposal uses three different kinds of resins including a crystalline polyester resin as binder resins of toner. However, trying to maintain the sea-island structure of the crystalline polyester resin results in an increase in dispersion diameter of the crystalline polyester, which raises problems of potentially leading to degradation of heat resistance storageability, causing transfer failures in a transfer step due to excessively lowered electrical resistance, and roughening the finally obtained image.
In still another proposal, a toner is provided with low-temperature fixing property and heat resistance storageability by defining the peak endothermic amount in a DSC curve measured with a differential scanning calorimeter to thereby control the existing state of a crystalline polyester resin for allowing the crystalline polyester resin to significantly exhibit its effects (see JP-A No. 2007-33773). In this proposal, however, the crystalline polyester resin is supposed to be used in combination with a non-crystalline polyester resin having a relatively high softening temperature. Therefore, the crystalline polyester resin is responsible for development of low-temperature fixing property, which naturally increases the amount of the crystalline polyester resin. As a result, the crystalline polyester resin and the non-crystalline polyester resin become in a compatible state to impair heat resistance storageability.
It has been proposed to incorporate into a toner an ester bond-containing crystalline polyester resin having a specific structure as a method for attaining low-temperature fixing property, heat resistance storageability and hot offset resistance at the same time (see JP-A No. 2005-338814). In this proposal, however, since the amount of the crystalline polyester resin incorporated is quite large, the crystalline polyester resin and the non-crystalline polyester resin turn into a compatible state, raising a problem that the heat resistance storageability is degraded.
In even another proposed method, a toner is defined in terms of the peak and the half width of its molecular weight distribution and of the amount of its chloroform insoluble matter, and two or more kinds of resins having different softening temperatures are used as a binder resin (see JP-B No. 4118498). In this proposal, however, since a crystalline polyester resin is not used, the low-temperature fixing property is inferior to a toner formed using a crystalline polyester resin.
As described above, at present, there is not a toner that is satisfactory in all of low-temperature fixing property, hot offset resistance and heat resistance storageability at the same time and in a balanced manner.